1. Field of the Invention
This invention relates to a rubber composition for a bead filler in a pneumatic tire, and more particularly to a rubber composition suitable for a tire bead filler having excellent durability, ride comfortability against vibrations, cornering stability and processability.
2. Related Art Statement
In pneumatic tires, particularly radial tires, the bead filler rubber largely affects the durability, cornering stability, ride comfortability and so on of the tire. For this end, various examinations have been made with respect to the bead filler rubber. For instance, the improvement of running performances and the like by applying a superhard rubber to a bead filler is disclosed in Japanese Utility Model Application Publication No. 47-16,084, French Patent No. 126,013 and U.S. Pat. No. 4,067,373. In these conventional techniques, however, the durability is substantially left out of consideration.
As a technique considering the durability, therefore, Japanese Patent laid open No. 55-54,337 discloses that a rubber composition comprising a diene rubber such as natural rubber or the like, a novolak type phenolic resin, a curing agent for resin and carbon black is used for the bead filler.
Further, Japanese Patent laid open No. 55-151,053 discloses a rubber composition for bead filler consisting mainly of a polymer obtained by block or graft polymerizing short fiber-like syndiotactic-1,2-polybutadiene with cis-1,4-polybutadiene, a thermosetting resin and a curing agent for improving both the durability and the running performances.
However, these latter conventional techniques have such a drawback that when the thermosetting resin and the curing agent for resin are added to the rubber composition for bead filler in order to enhance the elastic modulus of rubber, since hexamethylenetetramine is used as a curing agent, the strength of polyethylene terephthalate fiber cord is reduced through amine, resulting in the degradation of tire durability as disclosed in Rubber Chemistry and Technology, Vol. 49, No. 4, 1976, pp 1040-1059. On the other hand, when hexamethoxyl methylol melamine is used instead of hexamethylenetetramine as a curing agent for thermosetting phenolic resin, the reduction of strength in the polyethylene terephthalate fiber cord due to amine decomposition can be prevented, but the curing reaction of the resin is accompanied during the processing such as kneading, extrusion or the like, so that there are some problems in the uniform quality and processability.
Further, there is a method of compounding a large amount of carbon black for enhancing the elastic modulus. In this method, however, the fluidity of rubber is considerably degraded, so that there are many restrictions in usual processing equipments for rubber industry such as Banbury mixer, extruder or the like and it is difficult to obtain rubber having a desired high elastic modulus.
On the other hand, high elastic modulus rubber can be obtained by adding a large amount of a cross-linking agent for rubbery polymer such as sulfur or the like. In this case, the bending durability required for bead filler is considerably degraded, so that such an addition is of no practical use.
As mentioned above, the conventional techniques make impossible to provide the rubber composition for tire bead filler satisfying all of running performances, durability, processability and so on.